To control any of the properties for a layer that are found on the Base Heights,
Extrusion, or Rendering tabs of the layer property sheet in ArcScene, you can use the
I3DProperties interface which is implemented by several co-classes associated with the layer: Basic3DProperties, Feature3DProperties, Raster3DProperties, and Tin3DProperties.
These are known as layer extensions.

How to use:

Add a feature layer to ArcScene.

Paste the code into ArcScene's VB Editor.

Run the procedure.

Public Sub AccessLayer3DProperties()
' in ArcScene VBA...' get the documentDim pSxDoc As ISxDocument
Set pSxDoc = Application.Document
' get the Scene from the Document:Dim pScene As IScene
Set pScene = pSxDoc.Scene
' get a layer in the scene (assumes there is one)Dim pLayer As ILayer
Set pLayer = pScene.Layer(0)
' get layer extensions:Dim pLayerExts As ILayerExtensions
Set pLayerExts = pLayer
' get 3d properties from extension:' layer must have it if it is in scene:Dim i As Long
Dim p3DProps As I3DProperties
For i = 0 To pLayerExts.ExtensionCount - 1
If TypeOf pLayerExts.Extension(i) Is I3DProperties Then
Set p3DProps = pLayerExts.Extension(i)
Exit For
End If
Next' set the layer's base height property to an expression' that equals an item multiplied by a constant.' This case assumes we have a feature layer with an item' called [HEIGHT]
p3DProps.BaseOption = esriBaseExpression
p3DProps.BaseExpressionString = "[HEIGHT] * 3.28 "
' Tell the scene graph to invalidate the layer and redraw:Dim pSG As ISceneGraph
Set pSG = pScene.SceneGraph
pSG.Invalidate pLayer, True, True
pSG.RefreshViewers
End Sub